A variety of laser modules have been disclosed, and have been widely used in making layer sights and laser pointers, which commonly use a laser diode to produce a laser beam. FIGS. 1 and 2 illustrate two structures of laser module mounting device according to the prior art. These structures are similar, each comprising a lens holder (1b;1c) to hold a condensing lens (3b;3c), a fastening block (7b;7c) to fasten the condensing lens to the lens holder, a barrel (4b;4c) to hold a laser diode (5b;5c), a printed circuit board (6b;6c) connected to the laser diode for operation control, and a cylindrical casing (8b;8c) to hold the aforesaid components in place. By turning the lens holder (1b) or the barrel (4c) in either direction, the focus is adjusted. These structures of laser module mounting device are not satisfactory in function and have some drawbacks. Because the fastening block is fastened to the lens holder by a screw joint or an adhesive agent to hold the condensing lens in place, it is difficult to adjust the focus when the internal structure is vibrated to displace and to produce a deformed oval light spot. Further, because the laser diode and the printed circuit board are fastened inside the cylindrical casing in parallel with each other, it is difficult to adjust the focus when the laser diode or the printed circuit board is tiled.
Further, the configuration of the light spot of the laser beam of a laser diode may vary with different installation conditions. As illustrated in FIGS. 3 and 4, the laser beam from a laser diode 1a is in a conical shape 2a, and the image formed is an oval light spot 3a. An angle (normal side relief angle) .theta. is formed by connecting the two opposite ends a;a' of the major axis of the oval light spot 3a with the laser firing point. This angle is about 20.degree. to 40.degree.. An angle (normal end relief angle) .theta.' is formed by connecting the two opposite ends b;b' of the minor axis of the oval light spot 3a with the laser firing point. This angle is about 6.degree. to 12.degree.. Referring to FIG. 5, when a plano-convex lens 4a is disposed in front of the laser beam 11a, the laser beam is refracted into a cylindrical light beam 21a to form a circular light spot 31a on an object. Referring to FIG. 6, when a tube 6a is mounted around the laser diode and the plano-convex lens, a ring of light 7a will occur around the light spot 32a. Referring to FIG. 7, when the front end of the tube, which hold the laser diode and the plano-convex lens, is blocked with a small laser firing hole 8a left for passing the laser beam, the ring of light shown in FIG. 6 is eliminated, and therefore the image formed on an object is a circular light spot.